DESCRIPTION: [unreadable] Pulmonary infection with gram-negative bacteria, particularly Pseudomonas aeruginosa (PA), is the primary cause of death in cystic fibrosis (CF) patients. Control of PA involves intensive antibiotic therapy with intravenous and/or inhaled antibiotics. Inhaled administration achieves high drug concentrations in the lung with reduced systemic exposure. Aztreonam is very effective against PA, and its use should not contribute to the emergence of aminoglycoside-resistant strains of PA due to its mechanism of action. This Phase 3 trial seeks to determine the ability of inhaled aztreonam (AI) to maintain or improve the clinical status following a 28-day course of tobramycin solution for inhalation. Patients will receive a 28-day course of 75 mg AI or volume matched placebo twice or three times daily via eFlow electronic nebulizer. The primary endpoint is the time to need intravenous or inhaled antibiotic treatment from the start of AI/placebo based on the presence of at least one of four symptoms predictive of pulmonary exacerbation. [unreadable] [unreadable]